The present invention relates to an AGC (Automatic Gain Control) circuit, and in particular to an AGC circuit which is digitized so that it is appropriate for integration.
An AGC circuit is employed for wireless portable radio devices, such as radios, portable telephones, car telephones and cordless telephones, for example, wherein a received high-frequency signal is dropped to an intermediate-frequency signal by a local frequency, and the intermediate-frequency signal is amplified to acquire a signal having a constant amplitude, so that demodulation can be stably performed. That is, in the AGC circuit, the amplitude of a signal which is obtained by amplifying an intermediate-frequency signal is monitored, and the gain of an amplifier is so controlled that the amplitude of an output signal after amplification is constant.
FIG. 10 is a circuit diagram showing a common, conventional AGC circuit. This AGC circuit comprises: an amplifier 10 having a digitally controlled gain G; a rectifier 20 for rectifying an output Vout of the amplifier 10; a smoothing circuit 30 for smoothing an output Va of the rectifier 20; a comparator 36 for comparing an output Vb of the smoothing circuit 30 with a constant voltage Vc; and an up-down counter 40 for incrementing or decrementing a count in accordance with whether an output Vd of the comparator 36 is at level H or at level L. In accordance with an n-bit output Qn of the up-down counter 40, the resistance of a feedback resistor Rf in the amplifier 10 is changed and the gain G (=-Rf/Rin) of the amplifier 10 is controlled.
FIG. 11 is a waveform diagram showing the characteristic of the conventional AGC circuit in FIG. 10. As is shown in FIG. 11, a high-frequency signal Vin is input and amplified by the amplifier 10, and the signal Vout is generated and output. In this example, the amplifier 10 is an inversion amplifier having an input resistor Rin and a feedback resistor Rf. The gain G is -Rf/Rin, as is described above. The rectifier 20 employs a comparator 22 to determine whether the output signal Vout of the amplifier 10 has a positive voltage cycle or a negative voltage cycle, and permits only an output signal Vout having a positive voltage cycle to pass through a switch 23. A voltage Va that has passed through is integrated by the smoothing circuit 30, which serves as a filter and is constituted by a resistor 32 and a capacitor 34, and is transformed into a direct current voltage Vb.
The direct current voltage Vb is proportional to an average value for output signals Vout, and is compared with a predetermined constant voltage Vc by the comparator 36. When the DC voltage Vb is lower, a digital output Vd at level H is output, and when the DC voltage Vb is higher, digital output Vd at level L is output. In accordance with the level of the digital output Vd, a count is incremented or decremented by the up-down counter 40. For example, when the output Vd is at level H, the count is incremented, and the value of the feedback resistor Rf is adjusted by an output Qn to increase the gain G of the amplifier 10. However, when the output Vd is at level L, the count is decremented, and the value of the feedback resistor Rf is adjusted by the output Qn to reduce the gain G of the amplifier 10. As a result, the gain G of the amplifier 10 is so controlled that the output signal Vout amplitude is constant.
Since the conventional AGC circuit shown in FIG. 10 monitors the amplitude of an output signal Vout obtained by amplification and controls the gain of the amplification, analog circuits, such as a rectifier and a smoothing circuit, must be employed. In general, the size of these analog circuits tends to be larger than that of digital circuits.
In addition, as is shown in FIG. 11, the output Vb of the smoothing circuit 30 includes a ripple effect that fluctuates above and below a reference voltage Vc. When the input Vin is a low-frequency signal, the output Vout, which is obtained by amplifying the input Vin, is also a low-frequency signal, and the above described ripple effect is increased. In order to prevent this, at the smoothing circuit 30, which is a low-pass filter, a cutoff frequency fc (=1/2.pi.RC) must be reduced. To reduce the cutoff frequency fc, however, the resistance of the resistor 32 or the capacitance of the capacitor 43 must be increased.
When the resistance or the capacitance at the smoothing circuit 30 is increased, first, the size of the circuit is enlarged, which constitutes a barrier to integration. Second, the charging and the discharging in the smoothing circuit requires an extended period of time, and the response of the AGC circuit is deteriorated.
Therefore, an AGC circuit is demanded that is appropriate for integration while maintaining the response of the AGC circuit is demanded.